After the Gibb's Farquharson patent expired, Francotte and its subcontractors made a number of "Farquharson" style actions. Most of those actions are marked PD or ID on the face (hidden by forearm).
I believe most of these actions were imported to England where the gun manufacturing trade bfinished the guns and sold them to retailers like Jeffrey, Holland and others (including some somewhat obscure retailers). Those guns are only marked with the retailer's name. My guess is that S A is a retailer hat had their name added to teh gun.